Rapid reduction of aqueous selenate with chromous ions

ABSTRACT

Processes are provided for the kinetically efficient reduction of selenate species to selenide species using chromous ions in acidic solution. This reduction may advantageously be carried out in the presence of sulphate species, with selective selenate reduction in preference to the reduction of sulfate. The reduced selenate may be removed from the chromous-treated solution, for example by precipitation of a copper-selenide solid. The chromic ions formed by reaction of chromous ions in the reduction of selenate may also be removed from solution, for example by addition of a base to form an insoluble chromic hydroxide solid. The chromic hydroxide may be recycled to regenerate chromous ions, for example by electrolysis. In this way, systems are provided for continuously removing dissolved selenium from wastewater streams.

FIELD

Innovations are disclosed in the field of aqueous redox chemistry,including processes for removing dissolved selenium species fromwastewaters.

BACKGROUND

Selenium in a trace amount is an essential element that plays animportant role in humans and animals in preventing of diseases. As anutrient, selenium is used by human body at recommended doses of 70μg/day for men and 50 μg/day for women (Tinggi, 2003). However, whenselenium is released to the environment from both natural and industrialactivities it can lead to increased concentrations in surface water,groundwater, soils, and vegetation. Unfortunately, even smallconcentrations of selenium can be toxic for many forms of aquatic life.Additionally, selenium accumulation to higher concentrations in wildlifespecies tissues leads to toxic levels for aquatic life, birds, andmammals including humans. In the interest of protecting aquaticenvironments, selenium discharge into the environment is closelycontrolled and monitored. (Beatty and Russo, 2014; IAEA, 2007; Tinggi,2003)

Selenium in nature is mostly associated with metal sulfide minerals suchas copper, iron, zinc, and lead sulfides (Zingaro and Cooper, 1974). Thetreatment of these ores to extract the metals of interest has thepotential for selenium release and transport into surface andgroundwater systems. Other sources of selenium contamination includecoal mining, fossil fuel combustion, oil refining, and discharge ofseleniferous drainage water from agriculture (Zingaro and Cooper, 1974).Selenium contamination in the receiving environment is a key issue formany industries and the treatment of wastewater contaminated withselenium is a growing challenge.

Selenium speciation in solution plays an important role in its removalas well as toxicology especially at low levels. Selenite (SeO₃ ²⁻,Se(IV)) and selenate (SeO₄ ²⁻, Se(VI)) are the most important inorganicselenium species which are generally found in water and known to betoxic (Beatty and Russo, 2014). Relative to selenate, selenite can bequite easily removed from solutions using various treatment methods suchas chemical reduction (e.g., zero valent iron and sulfur dioxide), andprecipitation and adsorption by ferrihydrite salt. These methods are notefficient for selenate removal due to the high solubility of selenate inthe solutions, its weak adsorption on the surfaces of precipitates, andslow kinetics of selenate reduction (Murphy, 1988; Sandy and DiSante,2010). In Canada, the Canadian Council of Ministers of the Environment(CCME) has set Water Quality Guidelines, which for selenium are 1 μg/Lin fresh water. In 2014, British Colombia updated its own guideline at 2μg/L for fresh water and marine aquatic life (Beatty and Russo, 2014;CCME, 2009).

The redox potential for the selenate/selenite redox couple is quitehigh; therefore, the reduction of selenate to selenite (HSeO₄ ⁻ toH₂SeO₃ or SeO₄ ²⁻ to HSeO₃ ⁻) is thermodynamically favorable. However,kinetically, the reaction is known to be slow (Mokmeli et al., 2013;Sandy and DiSante, 2010). Consequently, reagents that are suitable toreduce selenite to elemental selenium or selenide are thermodynamicallyeffective for selenate reduction as well, but reaction times areunacceptably long. For instance, Weir et al. (1982) reported thatselenite can be quite easily reduced to elemental selenium using copperpowder (99% after 1 min in 10-50 g/I sulfuric acid at room temperature).In contrast, selenate reduction with metallic copper was much slower andrequired a higher temperature around the boiling point (Weir et al.,1983a; Weir et al., 1982).

Various reagents have been studied for the reduction of selenate to itslower oxidation state. For example, Koyama and Kobayashi (2000) usedcopper, iron, zinc and aluminum powder to reduce selenate in HClsolution at pH 2 and found that iron was the most efficient reductant.Zero valent iron has a high potential for the removal of selenate (up to100% in laboratory testing). However, the presence of certain salts likephosphate (PO₄ ³⁻) and nitrate (NO₃ ⁻) reduces the efficiency ofselenate removal to 43%. Mondal et al. (2004) reported that the presenceof 2.5 g/L sulphate in solution reduced the selenate removal by NiFepowder from 100% to 71.5% in a standard experiment. The otherdisadvantages of the use of metals in the reduction of selenate is thatusually a large excess of metal is required per unit of selenate in thesolution which increases the costs of reagents and introduces asignificant challenge in waste management of the treatment of sludge.For instance, Mondal et al. (2004) studied the reduction of selenate byNiFe powder and found the maximum selenate removal of 98% (10 min atinitial selenate concentration of 100 ppm) at a NiFe powder usage of 5g/L. However, at a lower NiFe powder usage (0.6 g/L), the selenateremoval decreased dramatically (80% after 2 h) and there was no furthersignificant selenate removal at a longer residence time. The reductionof selenate by sulfur dioxide has also been studied and it was foundthat the reaction requires an autoclave at higher temperature (above140° C.) and a long reaction time (several hours) is needed for acomplete reduction (Weir et al., 1983b; Zingaro and Cooper, 1974). Ingeneral, current methods available for selenate removal from watersolutions suffer from slow kinetics, incomplete removal of selenate, useof expensive reagents, high energy cost to heat solutions to hightemperature and secondary contamination of the treated solution.

SUMMARY

The invention provides a process for the reduction of selenate speciesto selenide species in solution. Surprisingly, the chromous ion rapidlyreduces selenate to selenide under acid conditions. Additionally (andalso surprisingly), the chromous ion reduces selenate selectively overthe reduction of sulfate where the aqueous solution contains bothspecies. The reduced species of selenate may be removed from thechromous-treated solution, for example by precipitation of acopper-selenide solid. The chromic ions formed by reaction of chromousions in the reduction of selenate may also be removed from solution, forexample by addition of a base to form an insoluble chromic hydroxidesolid. The chromic hydroxide may be recycled to regenerate chromousions, for example by electrolysis.

Aspects of the process involve reduction of dissolved aqueous selenate(Se(VI)) in an acidic reduction medium comprising reacting the selenatewith chromous (Cr(II)) ions. This reduction may for example take placeat an initial Cr(II)/Se(VI) molar ratio of 8 or above. Conditions may beprovide so that at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 99% or 99.5%of the selenate is reduced to a hydrogen selenide (H₂Se) product. Thisprocess may take place over a reaction time, which may for example be isless than 24, 20, 15, 10 or 5 hours, to provide a selenate-barrensolution having a final concentration of Se(VI). The initialconcentration of Se(VI) may for example be less than 100 ppm, 90 ppm, 80ppm, 70 ppm or 60 ppm. The initial concentration of Se(VI) may also begreater than 50 ppm, 40 ppm, 30 ppm, 20 ppm or 10 ppm. The finalconcentration of Se(VI) may for example be less than 5 ppm, 4 ppm, 3ppm, 2 ppm or 1 ppm.

The reduction medium may include a dissolved sulphate, present forexample in a concentration of greater than 0.1 mole/L, or less than 4mole/L, or 0.1 to 4 mole/L, or greater than about 10 g/L, or less thanabout 400 g/L, or about 10 to 400 g/L. As exemplified, conditions may beprovided so that the reduction by the chromous ions in the reductionmedium is selective for the dissolved selenate over the dissolvedsulphate.

The process may for example be carried out at an ambient temperature,below the boiling point of the reduction medium, at 10° C.-30° C., or atabout 25° C.

The pH of the acidic reduction medium may for example be less than 2.2,2.1, 2, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1 or 1. Alternatively,the pH of the reduction medium may be greater than 2.2, and in that casethe process may be carried out above the ambient temperature, forexample at an elevated temperature that is below the boiling point ofthe reduction medium.

The reduction of selenate by chromous ions to form the hydrogen selenideproduct may accordingly involve the reaction:8CrSO₄+H₂SeO₄+4H₂SO₄=H₂Se+4H₂O+4Cr₂(SO₄)₃.

The process may involve generating chromous ions by a chemical or anelectrochemical reduction of a chromic solution. Electrochemicalreduction may for example involve reducing a solution of potassiumchromium(III) sulfate dodecahydrate (KCr(SO₄)₂.12H₂O), and the reductionof the KCr(SO₄)₂.12H₂O may for example be carried out in a dividedelectrochemical cell. The exemplified divided electrochemical cellincluded a graphite felt cathode supported by a titanium mesh, a Nafionmembrane separator between an anolyte and a catholyte, and an anodeformed from a lead-silver alloy. The electrochemical cell may have ananolyte compartment containing a sulfuric acid solution, with thereactions carried out in the divided electrochemical cell being:

Cathode: 2KCr(SO₄)₂ + 2e + 2H⁺ = 2CrSO₄ + K₂SO₄ + H₂SO₄ Membrane: 2H +(anolyte) = 2H + (catholyte) Anode: H₂O = 2H⁺ + 1/2O₂ + 2e Overall2KCr(SO₄)₂ + H₂O = 2CrSO₄ + K₂SO₄ + H₂SO₄ + 1/2O₂. Reaction:

The hydrogen selenide product may be removed from the reduction medium,for example by contacting the reduction medium with a chemical species,such as a copper species, to carry out a precipitation reaction thatforms an insoluble precipitate. The copper species may for example be asoluble copper salt and/or a copper containing oxide material. If thecopper containing oxide material is CuO, the precipitation reaction maybe carried out at a pH above 6 to form solid cupric selenide (CuSe), andthe precipitation reaction may be carried out at a CuO/H₂Se molar ratioof 10 or higher. If the copper containing oxide material is Cu₂O, theprecipitation reaction may be carried out at pH 3.5 to form solidcuprous selenide (Cu₂Se), and the precipitation reaction may be carriedout at a Cu₂O/H₂Se molar ratio above 6. Wherein the soluble copper saltis copper sulfate (CuSO₄), the precipitation reaction may be carried outat pH above 4, to form the solid CuSe, and the precipitation reactionmay be carried out at a CuSO₄/H₂Se molar ratio above 6. The removal ofthe hydrogen selenide from the reduction medium may accordingly involveone or more of the following reactions:H₂Se+CuSO₄=CuSe+H₂SO₄H₂Se+CuO=CuSe+H₂OH₂Se+Cu₂O=Cu₂Se+H₂O.

Alternatively, the hydrogen selenide may be removed by from thereduction medium by sparging with a purging gas, such as N₂ or an inertgas. The sparged hydrogen selenide may for example be captured in ascrubbing reaction, which may involve reacting the sparged hydrogenselenide with Cu₂O and/or CuO.

Chromic ions formed in the reduction medium by the oxidation of chromousions may be removed from the reduction medium by a pH adjustment, forexample by addition of a base to form an insoluble chromic hydroxide.The base may for example be sodium hydroxide or aqueous ammonia, and thepH adjustment to form the insoluble chromic hydroxide may accordinglyinvolve the reaction:Cr₂(SO₄)₃+6NH₄OH=2Cr(OH)₃+3(NH₄)₂SO₄.

The insoluble chromic hydroxide may be recovered as a solid andre-dissolved with sulfuric acid. With an addition of potassium sulfate,the original solution for electrochemical synthesis can be re-generated:Cr(OH)₃+1.5H₂SO₄+½K₂SO₄═KCr(SO₄)₂+3H₂O.

The KCr(SO₄)₂ product may be directed to a solution for electrochemicalsynthesis of regenerated chromous ions and the regenerated chromous ionsmay be recycled to the reduction reaction.

The reduction medium may be made up from an ion exchange eluant from anion exchange process, with the selenate-barren solution recycled back tothe ion exchange process. The ion exchange process may for exampleinvolve:

-   -   passing a primary aqueous solution comprising selenate over an        anion exchange resin, such as a strongly basic resin, under        conditions whereby the selenate binds to the resin to produce a        selenate-loaded resin and an ion exchange discharge solution        comprising a lower concentration of selenate than the primary        aqueous solution; and,    -   treating the selenate loaded resin with a regenerant solution        comprising the selenate-barren solution under regenerating        conditions whereby a second anion in the regenerant solution        displaces selenate anions from the selenate-loaded resin to        produce a selenate-laden regenerant solution having a higher        concentration of selenate than the primary aqueous solution,        wherein the selenate-laden regenerant solution is directed to        form a component of the acidic reduction medium.

The primary aqueous solution may for example be a wastewater comprisingdissolved sulphate, so that the second anion is sulphate.

The disclosed processes may for example be carried out essentiallycontinuously. As such, systems are provided for continuously removingdissolved selenium species from an input water stream comprising adissolved selenate, wherein the system removes the dissolved selenate byreduction of the dissolved selenate with chromous ions to form hydrogenselenide, wherein the chromous ions comprise recycled chromous ionsproduced by the system. As a result, such a system may be adapted toproduce hydrogen selenide from the dissolved selenate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an Eh-pH diagram of selenium at 25° C. and selenium activityof 10⁻⁴ M. (derived from HSC Chemistry 6)

FIG. 2 is a schematic illustration of the exemplified setup forgeneration of chromous solution. 1) Anode compartment, 2) Cathodecompartment, 3) Cation exchange membrane, 4) Anode electrode, 5) Cathodeelectrode, 6) Argon sparging inlet, 7) Circulation tube, 8) Samplingport.

FIG. 3 is a schematic illustration of a stoichiometry example set-up.

FIG. 4 is a schematic illustration of a Stopped-Flow setup used inkinetics examples.

FIG. 5 is a line graph illustrating current efficiency and conversion ofCr(III) to Cr(II) as a function of time. Initial catholyte: 0.1 MCr(III) as sulfate and 0.05 M K₂SO₄ and 0.1M H₂SO₄.

FIG. 6 is a line graph illustrating the stoichiometry of the selenatereduction with chromous ions at pH 1 and 20° C.

FIG. 7 is a line graph illustrating In [Se(VI)]-In [Se(VI)]₀concentration as a function of time at [Cr(II)]₀=0.047 M,[Se(VI)]₀=0.00022 M, 20° C., and an ionic strength of 1 M.

FIG. 8 is a line graph illustrating selenate concentration as a functionof time at [Cr(II)]₀=0.047 M, [Se(VI)]₀=0.00022 M, pH 1.0, 20° C., andan ionic strength of 1 M.

FIG. 9 is a line graph illustrating selenate concentration as a functionof time at different pH, [Cr(II)]₀=0.047 M, [Se(VI)]₀=0.00022 M, 20° C.and an ionic of strength of 1 M.

FIG. 10 is a line graph illustrating selenate concentration as afunction of time at different chromous concentrations, pH 1.0, 20° C.,an initial selenate concentration of 0.00022 M and an ionic strength of1 M.

FIG. 11 is a line graph illustrating selenate concentration as afunction of time at different temperatures, pH 1.6, 0.047 M Cr(II), aninitial selenate concentration of 0.00022 M and an ionic strength of 1M.

FIG. 12 is a line graph illustrating In([Se(VI)]-In([Se(VI)]₀ as afunction of time and different sulfate concentrations at [Cr(II)]₀=0.026M, [Se(VI)]₀=0.00022 M, 20° C. pH 1 and an ionic of strength of 1 M.

FIG. 13 is a line graph illustrating In([Se(VI)]-In([Se(VI)]₀ as afunction of time and different sulfate concentrations at [Cr(II)]₀=0.047M, [Se(VI)]₀=0.00022 M, 20° C. and pH 2.2.

FIG. 14 is a line graph illustrating hydrogen removal as a function oftime at CuO/H₂Se ratios of 10, 15 and 25, an initial H₂Se concentrationof 0.0022 M, 20° C. and pH 3.5.

FIG. 15 is a line graph illustrating hydrogen removal as a function oftime at a CuO/H₂Se ratio of 10, an initial H₂Se concentration of 0.0022M, 20° C. and different pHs.

FIG. 16 is a line graph illustrating hydrogen selenide removal as afunction of time at a CuSO₄/H₂Se molar of 6, 22° C., an initial H₂Seconcentration of 0.0022 M, 20° C. and different pHs.

FIG. 17 is a line graph illustrating hydrogen selenide removal as afunction of time at a Cu₂Se/H₂Se molar of 6 and 15, 22° C., an initialH₂Se concentration of 0.0022 M, 20° C. and pH 2.

FIG. 18 is a line graph illustrating hydrogen selenide removal as afunction time at pH 2 and 3.5, an initial H₂Se concentration of 0.0022M, 20° C. and Cu₂₀/H₂Se molar ratio of 6.

FIG. 19 is a flow sheet of the selenate removal from ion-exchange eluantwith the use of chromous ions.

DETAILED DESCRIPTION

As disclosed in the following Examples, it has been discovered that thechromous ion can, surprisingly quickly and completely, reduce selenateand form hydrogen selenide as a reaction product. For use in thisreduction process, chromous ions can for example be generated bychemical or electrochemical reduction of chromic solution. The resultinghydrogen selenide can be removed from solution, for example by furtherreaction with copper species. It has also been shown that the chromicions formed by the reaction of chromous ions can be removed fromsolution by pH adjustment.

A solution of chromous ion can be prepared by chemical orelectrochemical reduction. For example, a solution of potassiumchromium(III) sulfate dodecahydrate (KCr(SO₄)₂.12H₂O) can be reduced ina divided electrochemical cell with a graphite felt cathode supported bya titanium mesh, a Nafion membrane separator between the anolyte andcatholyte and an anode formed from a lead-silver alloy. The anolytecompartment contains sulfuric acid solution.

The main reactions in the electrochemical cell are:Cathode: 2KCr(SO₄)₂+2e+2H⁺=2CrSO₄+K₂SO₄+H₂SO₄Membrane: 2H⁺(anolyte)=2H⁺(catholyte)Anode: H₂O=2H⁺+½O₂+2eOverall Reaction: 2KCr(SO₄)₂+H₂O=2CrSO₄+K₂SO₄+H₂SO₄+½O₂

The chemical reduction of selenate by chromous ions to form hydrogenselenide can be written in the following reaction (for example).8CrSO₄+H₂SeO₄+4H₂SO₄=H₂Se+4H₂O+4Cr₂(SO₄)₃

The precipitation of selenide from solution may be accomplished bycontact with a chemical species that forms an insoluble precipitation.For example a soluble copper salt or a copper containing oxide materialmay be used to react with selenide to form a copper-seleniumprecipitate. Some example reactions are shown below.H₂Se+CuSO₄=CuSe+H₂SO₄H₂Se+CuO=CuSe+H₂OH₂Se+Cu₂O=Cu₂Se+H₂O

The precipitation of chromic ion from solution with base addition formsinsoluble chromic hydroxide. Suitable bases may be (for example) sodiumhydroxide or aqueous ammonia. Lime is generally not suitable as it formscalcium sulfate as a reaction product which contaminates the recoveredchromic hydroxide.Cr₂(SO₄)₃+6NH₄OH=2Cr(OH)₃+3(NH₄)₂SO₄

The chromic hydroxide can be recovered as a solid and re-dissolved withsulfuric acid. With an addition of potassium sulfate, the originalsolution for electrochemical synthesis can be re-generated.Cr(OH)₃+1.5H₂SO₄+½K₂SO₄═KCr(SO₄)₂+3H₂O

As exemplified herein, chromous ions are surprisingly efficient as areducing agent for selenate removal from waste water. As one example ofthe application of the exemplified selenate removal process, BioteQ(BioTEQ, 2018) discloses an application with selenium in waste waterbefore ion exchange (IX) treatment to recover selenate. In this process,the loaded ion exchange resin is eluted with a concentrated sulphatesolution to produce an eluant containing Se as selenate. The eluantneeds to be treated to remove selenium in order to recycle the eluant.The invention has been shown to be effective at removing selenate froman eluant of this kind.

Although various embodiments of the invention are disclosed herein, manyadaptations and modifications may be made within the scope of theinvention in accordance with the common general knowledge of thoseskilled in this art. Such modifications include the substitution ofknown equivalents for any aspect of the invention in order to achievethe same result in substantially the same way. Numeric ranges areinclusive of the numbers defining the range. The word “comprising” isused herein as an open-ended term, substantially equivalent to thephrase “including, but not limited to”, and the word “comprises” has acorresponding meaning. As used herein, the singular forms “a”, “an” and“the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a thing” includes more thanone such thing. Citation of references herein is not an admission thatsuch references are prior art to the present invention. Any prioritydocument(s) and all publications, including but not limited to patentsand patent applications, cited in this specification are incorporatedherein by reference as if each individual publication were specificallyand individually indicated to be incorporated by reference herein and asthough fully set forth herein. The invention includes all embodimentsand variations substantially as hereinbefore described and withreference to the Examples and drawings.

EXAMPLES

In order to provide examples of the optional steps in alternativeaspects of the present processes, a series of illustrative assays wereconducted. Specific embodiments were exemplified to demonstrate (1)chromous generation using an electrochemical cell, (2) stoichiometry ofthe chromous reaction with selenate, (3) kinetics of selenate reductionwith chromous, and (4) removal of hydrogen selenide from the solutionusing different reagents.

All chemicals used for the examples were reagent grade or higher gradewithout any further purification. In the chromous generation examples,potassium chromium(III) sulfate dodecahydrate (KCr(SO₄)₂.12H₂O) fromAlfa Aesar and 98% sulfuric acid from BDH were used to prepare catholyteand anolyte solutions. A Nafion N324 cation membrane from DuPont wasused to separate catholyte and anolyte. A 6 mm thick graphite felt fromCeraMaterials, USA was used as a cathode. Sodium selenate (Na₂SeO₄ 99%)from Sigma-Aldrich, sodium sulfate (Na₂SO₄) from Fisher Scientific,perchloric acid (HClO₄, 61.2%) from Fisher Scientific, sodiumperchlorate (NaClO₄.H₂O HPLC grade) from Fisher Scientific and sodiumhydroxide from Fisher Scientific were used to prepare solutions for thekinetics study. Cupric oxide powder (CuO, 350 mesh) from Sigma Aldrich,cuprous oxide (Cu₂O, 75 μm or less) from Alfa Aesar and copper sulfatefrom Fisher Scientific were used for the hydrogen selenide removalexamples.

NH₄Fe(SO₄)₂.12H₂O from Sigma-Aldrich was used to prepare ferric solutionto oxidize chromous and hydrogen selenide to chromic and selenium,respectively. Standard cerium (IV) sulfate (0.1N) solution from AlfaAesar was used to titrate ferrous ion from this reaction as an indirectindication of the content of chromous or hydrogen selenide reacted.Ultra-high-purity argon 99.999% Ar (<3 ppm 02) was used to purge thereactors and solutions to remove oxygen that would be expected to reactwith chromous and impact the reaction chemistry and kinetics.

Example 1. Generation of Chromous Ions

The electrochemical generation of chromous ions was conducted to producechromous solution for the selenate reduction examples. The currentefficiency and the energy consumption of reduction of chromic tochromous as a function of time was calculated.

Chromous solution was generated by the electrochemical reduction ofCr(III) in a three compartment cell with two anode compartments and onecathode compartment as shown in FIG. 2.

The electrochemical cell consisted of four blocks and was constructedentirely of chlorinated polyvinyl chloride sheets. The anode and cathodechambers had a capacity of 500 ml (each anode) and 1 L, respectively.The cathode compartment was equipped with three ports that were used forpurging argon gas, circulating the catholyte in the cathode compartmentand collecting the generated chromous solution into a storage container.The circulation of chromous solution through the cathode was performedusing a Masterflex pump at a rate of 100 mL/min to enhance the masstransfer. The superficial surface area of each of the anodes and thecathode were 114 cm².

Two titanium mesh frames were used to support and compress the graphitefelt between the two cathode compartments and to conduct electricity tothe graphite felt. The titanium surface was painted using achemical-resistant paint except for the electrical contact area with thegraphite felt. This was used to reduce the rate of formation of hydrogenon the titanium surface.

A Nafion N324 cation membrane (DuPont Inc.) was used to separate thecatholyte and anolyte. Two lead-silver alloy electrodes (1% Ag) wereused as anodes while a 6-mm thick graphite felt electrode(CeraMaterials, USA) was used as a cathode. The graphite felt cathodewas first washed using deionized water to remove any dust and then driedin the oven at 110° C. for 24 hours to activate the graphite surface(Hong, 2015; Sun and Skyllas-Kazacos, 1992).

The anolyte was prepared as a 0.1 M sulfuric acid solution usingconcentrated sulfuric acid solution and DI water. The catholytecontained 0.1M chromium (III) and 0.1 M sulfuric acid solution and wasprepared using KCr(SO₄)₂.12H₂O and sulfuric acid.

Each anode compartment was filled with the prepared anolyte solutionusing a volumetric cylinder. The catholyte was transferred into thecathode compartment using a peristaltic pump. The catholyte was purgedwith ultra-pure argon gas at least for half an hour to remove oxygenfrom the solution and headspace of the cell before starting the assay. Aslow flow of argon gas was maintained across the headspace of the cellduring the example to minimize oxygen ingress. A DC power supply from GWInstek was used to apply a current density of 200 A/m² to the graphitefelt. A logging multimeter and a standard resistor (5 A, 100 mV) wereused to ensure that the currents flowing through the two anodes were thesame. The electrolysis time was sufficiently long to ensure that almostall Cr³⁺ was converted to Cr²⁺. The generated chromous solution with theconcentration of 0.1 M Cr²⁺ and 0.15 M of H₂SO₄ was then transferredinto a glass container using a peristaltic pump under an argon gasatmosphere. The chromous solution was stored in a glass container underargon atmosphere to ensure no oxidation of chromous ions occurred by airingress. The analysis of this solution after one months of storageshowed that the chromous concentration did not change.

The concentration of generated chromous solution was measured by thepotentiometric titration method using Radiometer ABU 80 automaticburette. A known mass of chromous solution was mixed with a known massof 0.15 M ferric solution under an argon atmosphere. The ferric solutionwas made by dissolving (NH₄)Fe(SO₄)₂.12H₂O salt into 2 M sulfuric acidsolution and then purging with argon gas for 30 minutes to remove oxygenbefore introducing the chromous solution into it. Mixing with ferricsolution, Cr(II) was converted to Cr(III) while Fe(III) was reduced toFe(II), as shown below. The concentration of ferrous ions was determinedby titration with Ce(IV) as shown below.Fe³⁺+Cr²⁺=Cr³⁺+Fe²⁺Fe²⁺+Ce⁴⁺=Ce³⁺+Fe³⁺

The endpoint was indicated by addition of one drop of ferroin (5×10⁻⁶ M)into the solution. Blank titration was used to correct for indicatorerror.

The current efficiency of chromic reduction in 0.1 M sulfuric acidmedium was investigated under the conditions summarized in Table 1.

TABLE 1 Conditions for the electrochemical reduction of Cr(III) toCr(II) Temperature 25° C. Current density 200 A/cm² Cathode thickness 6mm Catholyte 0.1M fresh KCr(SO₄)₂ + 0.1M H₂SO₄ anolyte 0.1M H₂SO₄

The conversion and current efficiency of Cr(III) to Cr(II) as a functionof time are shown in FIG. 5. In the first 50 minutes, the conversion ofCr(III) to Cr(II) almost linearly increased to 60% while the currentefficiency only decreased from 83.3 to 82.9%. After 50 minutes, theconversion of Cr(III) to Cr(II) increased slowly and reached almost 100%at 160 minutes. The current efficiency decreased from 82.9% to 43% withincreasing time from 50 to 160 minutes. The energy consumption firstincreased slowly (from 3.41 to 3.83 kWh/kg in 80 minutes) as the currentefficiency decreased slightly and then increased significantly withdecreasing current efficiency. It finally reached 6.89 kWh/kg. In theconversion range of 80 to 90% which is sufficient for industrialapplications, the current efficiency was 81 to 76% while the energyconsumption was 3.5 to 3.83 kWh/kg.

Example 2. Selenate Reduction by Chromous Ions

The stoichiometry of the selenate reduction reaction by chromous ionswas investigated at different initial concentrations of chromous ionsand selenate and different pHs. All the assays were performed at roomtemperature. The initial chromous concentration in all tests was 0.05 Mand the molar ratio of chromous ions to selenate was adjusted by varyingthe initial concentration of selenate ions.

To perform each stoichiometry example, a 60 mL syringe with its topsealed with a septum was used as a reactor. The schematic illustrationof the set-up is shown in FIG. 3. The sealed syringe was chosen as areactor to ensure that there was no free headspace in the reactor toallow hydrogen selenide gas to escape from the solution into theheadspace. A magnetic stirring bar was placed in the syringe to agitatethe solution. Prior to starting the examples, the syringe reactor wasflushed with argon gas to remove the air and then placed in a glasscylinder. Both the head and bottom of the glass cylinder were sealedwith threaded plastic caps and a slow flow of argon was maintainedacross the cylinder during the exemplified assay. Argon gas was passedthrough the glass cylinder to prevent possible diffusion of oxygen intothe syringe and subsequently to prevent the possible oxidation ofchromous ions and H₂Se by oxygen.

The selenate solutions were prepared by dissolving Na₂SeO₄ in deionizedwater at the desired concentrations and then purging with ultra-highpurity argon gas for 30 minutes to remove oxygen from the solution.Sodium sulfate and sulfuric acid were also added as required to ensurethe solution pH reached the required value.

To conduct a selenate reduction example, a 60-mL reactor syringe wasfirst flushed with ultra-high purity argon gas and then filled with aknown mass of chromous solution through the septum. A known amount ofselenate solution was then quickly injected into the syringe reactorthrough the septum and rapidly mixed with the chromous solution using amagnetic stirring bar. The solutions were added on a mass basis using asealed container to minimize the oxidation of the chromous ions incontact with air. The densities of all the solutions were measuredbefore and after purging argon gas to calculate the volumes of thetransferred solutions.

A series of samples were taken for Cr(II), H₂Se and Se(VI) analysis. Forthe examples with an initial molar ratio of Cr(II)/Se(VI) over 8 (i.e.,an excess of Cr(II) present), half of each sample was immediately mixedafter collection with a known mass of 0.3 M ferric sulfate and 2 Msulfuric acid solution to convert both Cr(II) and H₂Se to Cr(III) andSe, respectively, while Fe(III) was reduced to Fe(II). The totalconcentration of H₂Se and Cr(II) was analyzed by titration of producedFe(II) with Ce(IV) using a Radiometer ABU 80 automatic burette. Theother half of the sample was immediately mixed with a sufficient amountof 10 M NaOH. The Cr(II) was precipitated as Cr(OH)₂ while H₂Se wasconverted to HSe⁻ and Se²⁻. In the acidic solution, the redox potentialof H⁺/H₂ is 0.42 V higher than that of Cr²⁺/Cr³⁺. However, hydrogen ionsdo not react with chromous ions because their reaction is kineticallyinhibited. This is consistent with the work by Jalan et al. (Jalan etal., 1985). However, at a high pH, chromous ions were precipitated asCr(OH)₂ and then reacted with water to generate hydrogen gas. Thehydrogen evolution was much faster in the presence of H₂Se, indicatingthat the reaction was catalyzed by H₂Se.2Cr(OH)₂+2H₂O=2Cr(OH)₃+H₂

After collecting all produced hydrogen gas, 5 M H₂SO₄ solution was thenadded to the mixture to dissolve chromic hydroxides and convertHSe⁻/Se²⁻ back to H₂Se species. The resulting solution, now free ofchromous ions, was mixed with a known mass of ferric solution. Mixingwith ferric solution, H₂Se was immediately converted to elemental Se,while Fe³⁺ was reduced to Fe²⁺. The solution was filtered through anOsmonics nylon 0.1 μm filter to remove elemental selenium and then thefiltrate was analyzed for ferrous concentration by the titration withCe⁴⁺ using a Radiometer ABU 80 automatic burette. The concentration ofchromous ions was calculated using the difference of the twomeasurements. The concentration of the remaining selenate in thesolution was measured using ICP-OES.

The stoichiometry of selenate reduction by chromous ions wasinvestigated at various initial molar ratios of chromous ions toselenate ions (from 9:1 to 43:1). The initial chromous concentration inall tests was 0.05 M and the molar ratio of chromous ions to selenatewas varied by changing the concentration of selenate ions. All theexemplified assays were conducted at pH 1 (except where noted), and 20°C.

As shown in FIG. 6, the average stoichiometric ratio of chromous ions toselenate was 8.07. With the consideration of experimental errors (e.g.,the oxidation of chromous ions by oxygen), slightly more chromous ionswere consumed. Therefore, the putative overall reaction is writtenbelow.SeO₄ ²⁻+8Cr²⁺+10H⁺=H₂Se+4H₂O+8Cr³⁺

Example 3. Kinetics of Selenate Reduction with Chromous Ions

The kinetics of selenate reduction by chromous ions were demonstrated byvarying the concentration of chromous ions, pH, sulfate concentration,ionic strength and temperature. All kinetics examples were conductedusing a stopped-flow apparatus.

To study the kinetics of a reaction, the time required to mix thereagents of the reaction should be negligible compared with the reactiontime. Since the reaction of chromous ions with selenate is very fastunder some conditions, the stopped-flow technique was used to study thereduction of selenate by chromous ions. A stopped-flow device is anapparatus for the rapid mixing of two solutions. Using the stopped-flowtechnique, the solutions are first forced from syringes into a mixingchamber. After several milliseconds, the observation cell is filled by apiston linked to a sensing switch that triggers the measuring device(e.g., a spectrometer) and the flow is stopped suddenly. Sincecommercial stopped-flow instruments are very expensive and usually usedfor mixing very small volumes of solutions (usually less than 1 mL), thestopped-flow device shown in FIG. 4 (the schematic illustration) wasfabricated for testing.

In this example, a UV-spectrometer was initially used to monitor theconcentrations of Cr(II) and Cr(III) ions in the solution. A largebackground interference with the detection of Cr(II) and Cr(III) by UVwas observed, which was most likely due to the formation of H₂Se.Therefore, the ICP-OES analysis was used for analyzing the selenateconcentration. A series of samples had to be taken and a large volume ofsolution (about 50 mL) was needed in each test, which was larger thanthe capacity of commercially available stopped-flow devices. Using thedesigned stopped-flow device (FIG. 4), 30 mL of chromous solution wasmixed with 30 mL of selenate solution to produce 60 mL of mixedsolution, providing enough solution samples.

As shown in FIG. 4, with the use of a pneumatic cylinder, equal volumesof selenate and chromous solutions were driven from syringes into amixer in less than 5 s and then the mixture immediately flowed into aglass syringe reactor to initiate the reaction. The head and bottom ofthe glass syringe reactor were sealed and also purged with argon toeliminate oxygen ingress and subsequent oxidation of chromous ions andH₂Se by oxygen. All the tubing connected to the reactor were flushedwith argon gas prior to running the exemplary embodiment in order tominimize oxygen in the system. The syringe reactor was immersed in awater bath to maintain a constant temperature.

Prior to running each exemplary embodiment, the syringes containingselenate and chromous solutions and sampling syringes were placed inglass containers filled with deoxygenated water under an argonatmosphere. The glass containers were immersed for over 30 minutes inthe water bath to ensure that the temperature of two solutionsstabilized at the target value before initiating the reaction.

Sampling from the solution was accomplished by applying a positivepressure of argon gas to the reactor and forcing the solution to flowthrough a needle into a glass tube containing a known mass ofapproximately 0.3 M ferric solution to stop the reactions. The glasstube was flushed with argon gas before introducing the sample to removeoxygen. The ferric solution was also deaerated with argon gas for 1 houror longer before mixing with the sample. The mixture of the sample andferric solution was shaken and heated to about 60° C. to accelerate theoxidation of H₂Se with ferric ions. The selenium concentration of eachsample was determined by ICP-OES analysis. Selenate solution wasprepared according to the required solution composition using sulfuricacid, perchloric acid, sodium perchlorate and sodium sulfate.Considering the method detection limit of ICP-OES for selenium (>100ppb), the initial selenate concentration was selected as 0.00022 M (17ppm) so that when 99% or less of selenate is removed from the solution,the selenate concentration can still be accurately determined. Despitethe fact that a higher initial selenate concentration is better for thedetermination of selenate concentration by ICP-OES, at a higherconcentration of selenate, the concentrations of chromous and hydrogenions will change more as the reaction proceeds and they cannot beconsidered to be constant. The concentration of chromous was also chosenat a sufficiently high value compared to the selenate so that thechromous concentration can be considered as constant when selenate iscompletely consumed. The HSO₄ ⁻/SO₄ ²⁻ system was used as a pH bufferingpair to maintain a constant pH over the course of reaction. Sodiumperchlorate was added to maintain a constant ionic strength.

The selenate solution was purged with ultra-high purity argon for 30minutes to remove oxygen from solution before mixing with the chromoussolution. The chromous solution for each exemplary assay was made asneeded by diluting the original chromous solution, 0.1 M Cr(II) (instock), with deoxygenated deionized water in a 50-mL flask while purgingthe headspace of the flask with ultra-high purity argon gas to maintainthe oxygen-free atmosphere. Based on the chromous solution composition(i.e., sulfate and proton concentrations), the selenate solution wasprepared in such a way that the initial composition of the mixturereached the targeted value. The concentrations of all the solutions werevalidated by chemical analysis (ICP-OES and titration), and in all casesagreed well with the theoretical dilution values.

FIG. 7 shows a plot of In ([Se(VI)]/[Se(VI)₀]) against time. The plot islinear with a slope of −0.0695, indicating that the reaction order withrespect to selenate concentration is 1. The selenate concentration as afunction of time is shown in FIG. 8. The selenate concentrationdecreased from 17 ppm to 1 ppm in 40 minutes.

The selenate concentration as a function of time at pH 1.0, 1.3, 1.6,1.9 and 2.2 are shown in FIG. 9.

The selenate concentration as a function of time at different chromousconcentrations are given in FIG. 10.

The selenate concentration as a function of time at various temperaturesis shown in FIG. 11.

The effect of sulfate on the rate of selenate reduction by chromous ionswas investigated by varying the concentration of sulfate from 0 to 0.15M at a chromous concentration of 0.026 M, selenate concentration of0.00022 M, an ionic strength of 1, pH=1 and at 20° C. The plots ofIn([Se(VI)]-In([Se(VI)]₀ against time at different sulfateconcentrations (FIG. 12) give straight lines which virtually overlapped.

Since the kinetics data also show that chromous ions does not reducesulfate, the use of chromous ions may be useful for the selectiveremoval of selenate in high sulfate concentration solutions.

In the use of ion exchange for selenate adsorption, theselenate-containing eluant can have a significant sulfate concentration(e.g. 1 M sodium sulfate or 1 M ammonia sulfate). Therefore, it isnecessary to conduct the selenate reduction by chromous ions in thesolution containing higher sulfate concentrations. At pH above 2.2, theselenate reduction is very slow. However, at a pH below 2.2, moresulfuric acid has to be used to adjust the pH to a lower value. Hencethe selenate reduction was carried out at pH 2.2, an initial chromousconcentration of 0.047 M, and an initial selenate concentration of0.00022 M, and 20° C.

The plots of In([Se(VI)]-In([Se(VI)]₀ against time at different sulfateconcentrations (FIG. 13) give linear lines which virtually overlapped.

Example 4. Hydrogen Selenide Removal from Solution

Selenate is reduced to selenide using chromous ions, which formshydrogen selenide in the acidic solution (pH<4). Therefore, the producedhydrogen selenide needs to be removed from the solution. All exemplifiedembodiments of hydrogen selenide removal were carried out in a similarmanner. The hydrogen selenide solution was first produced by thereduction of selenate by chromous ions and then CuO or Cu₂O solids orCuSO₄ solution were introduced to the solution to remove H₂Se from thesolution as CuSe or Cu₂Se.

A typical example first involved transferring of previously producedchromous solution into a reactor. A known amount of selenate solutionwas mixed with the chromous solution. The chromous ions were present ina quantity slightly higher than stoichiometrically required tocompletely reduce selenate ions to hydrogen selenide so that there wasnearly no chromous ion left in the solution after the completion of thereactions.

To measure the concentration of produced hydrogen selenide, a sample wascollected at the end of each reduction example and mixed with asufficient amount of 10 M NaOH to precipitate any remaining Cr(II) asCr(OH)₂ and convert H₂Se to HSe⁻ and Se²⁻. Cr(OH)₂ reacts with water toproduce Cr(OH)₃ and H₂. A 5 M H₂SO₄ solution was added then to themixture to dissolve chromic hydroxides and convert HSe⁻/Se₂ ⁻ back toH₂Se species. The resulting solution, now free of chromous ions, wasmixed with a known mass of ferric solution to measure the concentrationof H₂Se through titration with Ce⁴⁺ as explained previously. Theconcentration of the remaining selenate in the solution was measuredusing ICP-OES.

A typical hydrogen selenide removal example was conducted bytransferring 25 ml of previously produced hydrogen selenide solutioninto a sealed vessel, typically a 30-mL Pyrex bottle. The vesselcontained three openings: one for addition of NaOH, CuO slurry, Cu₂Oslurry or copper sulfate solution depending on the test, the second onefor the pH probe, and the third one for purging argon gas. The reactorwas first purged with argon gas to remove air and then a small pressureof argon gas was maintained in the reactor.

The solution pH was adjusted to a desired value by adding deoxygenated0.1 M NaOH at the beginning of the exemplified assay, as required. ThepH was recorded at the beginning and end of each example. The exampleswere conducted under constant agitation using a magnetic stirrer and atroom temperature.

In order to create a slurry of copper oxide powder, 0.5 g of CuO or Cu₂Owas mixed with 50 mL of deionized water in a sealed container and thenthe slurry was purged with argon gas for 30 minutes or longer. To ensurehomogeneity of the slurry was added to the reactor, a known amount ofslurry was collected from the container using a 5-mL syringe while itwas rapidly stirred. Immediately following addition of the copper oxideslurry, several samples were taken using a syringe to monitor theconcentration of hydrogen selenide left in the solution. The sampleswere filtered through an Osmonics nylon 0.1 μm filter to remove solidparticles and then the filtrate was analyzed for hydrogen selenideconcentration by titration. Solids were collected using a membranefiltration system and washed with deionized water and dried in air forx-ray diffraction (XRD) analysis. All the examples were performed atroom temperature.

Hydrogen selenide removal was studied by the addition of differentamounts of cupric oxide at different pHs. All the examples wereperformed at room temperature, constant stirring speed and under anargon atmosphere. The results of hydrogen selenide removal at differentCuO/H₂Se molar ratios is shown in FIG. 14. The results of changing pH onthe hydrogen selenide removal with CuO is shown in FIG. 15.

The hydrogen selenide removal as a function of time at different pH,CuSO₄ to H₂Se molar ratio of 6, and an initial hydrogen selenideconcentration of 0.0022M are shown in FIG. 16.

FIG. 17 shows the hydrogen selenide removal efficiency as a function oftime at different molar ratios of Cu₂O to H₂Se from a 0.002 M H₂Sesolution at pH 2. It was observed that hydrogen selenide was quicklyremoved within the first 10 minutes and then hydrogen selenide removalincreased slightly over the time. Hydrogen selenide removal increasedfrom 92 to 97% with increasing Cu₂O/H₂Se molar ratio from 6 to 15.

Hydrogen selenide removal as a function of time at pH 2 and 3.5 and Cu₂Oto H₂Se molar ratio of 6 are shown in FIG. 18. At pH 2.0, only about 90%of the hydrogen selenide was removed while at pH 3.5 hydrogen selenidewas almost completely removed from the solution. As compared to thehydrogen selenide removal with CuO, using Cu₂O much better removalefficiencies were observed.

The integrated process for selenate removal from an ion exchange eluantis shown in FIG. 19.

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The invention claimed is:
 1. A process for reduction of dissolvedaqueous selenate (Se(VI)) in an acidic reduction medium comprisingtreating a solution having an initial Se(VI) concentration by reactingthe Se(VI) with chromous (Cr(II)) ions at an initial Cr(II)/Se(VI) molarratio of 8 or above, whereby at least 95% of the Se(VI) is reduced to ahydrogen selenide (H₂Se) product within a reaction time of less than 5hours, to provide a lowered-Se(VI) solution having a final Se(VI)concentration that is lower than the initial Se(VI) concentration. 2.The process of claim 1, wherein at least 99% of the Se(VI) is reduced tothe H₂Se product within the reaction time.
 3. The process of claim 2,wherein the reaction time is less than 4 hours.
 4. The process of claim3, wherein the initial concentration of Se(VI) is less than 100 ppm. 5.The process of claim 4, wherein the initial concentration of Se(VI) isgreater than 10 ppm.
 6. The process of claim 5, wherein the finalconcentration of Se(VI) is less than 5 ppm.
 7. The process of claim 6,wherein the reduction medium comprises a dissolved sulphate.
 8. Theprocess of claim 7, wherein the dissolved sulphate is present in aconcentration of about 10 to 400 g/L.
 9. The process of claim 8, whereinreduction by the chromous ions in the reduction medium is selective forthe dissolved Se(VI) over the dissolved sulphate.
 10. The process ofclaim 9, wherein the process is carried out at 10-30° C.
 11. The processof claim 10, wherein the pH of the acidic reduction medium is less than2.2.
 12. The process of claim 9, wherein the pH of the reduction mediumis greater than 2.2 and the process is carried out above the ambienttemperature and below the boiling point of the reduction medium.
 13. Theprocess of claim 1, further comprising generating the chromous ions by achemical or an electrochemical reduction of a chromic solution.
 14. Theprocess of claim 1, further comprising removing the hydrogen selenideproduct from the reduction medium.
 15. The process of claim 14, whereinhydrogen selenide is removed from the reduction medium by contacting thereduction medium with a precipitating chemical species to carry out aprecipitation reaction that forms an insoluble precipitate.
 16. Theprocess of claim 15, wherein the precipitating chemical species is acopper species.
 17. The process of claim 14, wherein the hydrogenselenide is removed by from the reduction medium by sparging with apurging gas to produce a sparged hydrogen selenide.
 18. The process ofclaim 1, wherein chromic ions formed in the reduction medium by theoxidation of chromous ions are removed from the reduction medium by achromic-removing pH adjustment.
 19. The process of claim 1, wherein thereduction medium comprises an ion exchange eluant from an ion exchangeprocess.
 20. The process of claim 19, further comprising recycling thelowered-Se(VI) solution back to the ion exchange process.
 21. Theprocess of claim 20, wherein the ion exchange process comprises: passinga primary aqueous solution comprising Se(VI) over an anion exchangeresin, under conditions whereby the Se(VI) binds to the resin to producea Se(VI)-loaded resin and an ion exchange discharge solution comprisinga lower concentration of selenate than the primary aqueous solution;and, treating the Se(VI) loaded resin with a regenerant solutioncomprising the lowered-Se(VI) solution of claim 1 under regeneratingconditions whereby a second anion in the regenerant solution displacesSe(VI) anions from the Se(VI)-loaded resin to produce a Se(VI)-ladenregenerant solution having a higher concentration of Se(VI) than theprimary aqueous solution, wherein the Se(VI)-laden regenerant solutionis directed to form a component of the acidic reduction medium.